The present invention relates to a phase transition mode liquid crystal display device using liquid crystals comprised essentially of a mixture liquid crystals of a nematic liquid crystal and a cholesteric liquid crystal, or of a nematic liquid crystal and a chiral nematic liquid crystal, or of these three liquid crystals (these liquid crystal mixtures will be referred to hereinafter as a nematic-cholesteric mixture liquid crystal.).
A phase transition mode liquid crystal display device using a nematic-cholesteric mixture liquid crystal exhibits a strong brightness and a high contrast because such a device does not need a polarizer which has been necessarily used in the conventional field effect mode nematic liquid crystal display device. The phase transition mode liquid crystal display device using a nematic-cholesteric mixture liquid crystal is classified into a p type and an n type in accordance with the polarity of the dielectric anisotropy of the nematic liquid crystal. FIG. 1 illustrates a light transmitting intensity-applied voltage curve for a phase transition mode liquid crystal display device using the n type nematic-cholesteric mixture liquid crystal. A transparent or weak light-scattering state exists in the initial state where no voltage is applied thereto. However, the transparent state is changed into a focal-conic state in which exhibits a strong characteristic when the voltage is applied thereto, and this strong light scattering state is kept for a long time even after removal of the applied voltage. In order to return to the initial state, the application of a high voltage with a high frequency is required.
The phase transition mode liquid crystal display device using a p type nematic-cholesteric mixture liquid crystal is classified into a horizontal alignment type and a homeotropic alignment type in accordance with the aligning treatment of the substrate. As used in this specification, the horizontal alignment type is defined as having a horizontal surface treatment which makes the long axis of the molecules of the liquid crystal in the vicinity of the surface of the substrate align in the horizontal direction, i.e., more or less parallel to the surface of the substrate, and the homeotropic alignment type is defined as having a homeotropic surface treatment which makes the long axis of the molecules of the liquid crystal in the vicinity of the surface of the substrate align in the vertical direction, i.e., more or less perpendicular to the surface of the substrate.
FIG. 2 illustrates a light transmitting intensity-applied voltage curve for the horizontal alignment type phase transition mode liquid crystal display device. A transparent or weak light-scattering state is presented due to the horizontal aligning treatment of the surface of the substrate when no voltage is applied thereto. However, the transparent state is changed into a focal-conic state which exhibits a strong light-scattering characteristic with an increase of the applied voltage, and continued increase of the applied voltage changes the focal-conic state into the light-transparent and homeotropic nematic state, which returns to the initial state with the decrease of the applied voltage in accordance with the hysteresis curve shown in FIG. 2.
FIG. 3 illustrates a light transmitting intensity-applied voltage curve for the homeotropic alignment type phase transition mode liquid crystal display device using a p type nematic-cholesteric mixture liquid crystal. A transparent state with little cloudiness is presented in the initial state when no voltage is applied thereto. However, the transparent state is changed into the focal-conic state which exhibits a strong light-scattering characteristic with an increase of the applied voltage, and continued increase of the voltage changes the focal-conic state into the light-transparent homeotropic nematic state. As illustrated in FIG. 3, a feature of the homeotropic alignment type phase transition mode liquid crystal display device exists in its hysteresis curve wherein the homeotropic nematic state is returned to the initial state through a light-scattering state as shown by dashed-line curve A when the voltage is suddenly cut off. On the other hand, the homeotropic nematic state transforms to the focal-conic state with a slow decrease of the applied voltage as shown by solid-line curve B. Finally the focal-conic state is maintained in spite of the absence of the applied voltage.
As described above, there are many kinds of phase transition mode liquid crystal display devices, and application research is actively underway in order that these devices can be put to practical use as display devices for computers, electric calculators, wristwatches, etc.
The phase transition mode liquid cristal display device can carry out the following two display operations. One is the display operation wherein a display background (the portion other than the selected picture elements) exhibits a transparent state and the selected picture elements exhibits a light scattering state (this type didplay operation will hereinafter be referred to as a positive display) and the other is the display operation wherein a display background a light-scattering state and the selected picture elements exhibit a transparent state (this type display operation will hereinafter be referred to as a negative display). These displays can be carried out by the application of the alternative voltages between the transparent electrodes on the substrates which are arranged so as to form the desired display pattern.
As compared with the two kinds of the display, the negative display is superior to the positive display in that it exhibits a more attractive display and has easier readability. However, in the conventional negative display, extra means for applying the voltage for keeping the focal-conic state is required in order to maintain the display background in the light-scattering state.
Therefore, the conventional negative display device has the disadvantages of too large a power consumption and complexity of the arrangement of the transparent electrodes.